Our bodies are made up of many cells, eg muscle, nerve, and skin cells. These cells use nutrients for energy, growth and repair. They also need oxygen to use in energy production by respiration . As they use the nutrients they produce wastes, eg carbon dioxide and urea. These have to be taken away from the cells and eliminated from the body. Our cardiovascular (heart-blood) system, respiratory system and excretory system work together to do this, as shown in the diagram below.

Respiration

The set of chemical reactions in which oxygen combines with glucose to form carbon dioxide and water, and release energy.
Glucose + Oxygen --> Carbon dioxide + Water + Energy